Method For Evaluating Hydraulic Fracturing

ABSTRACT

A method for evaluating hydraulic fracturing places distinctive tracers in each of the fractured regions around a number of production wellbores. For example, these tracers can by carried in the fracking fluid. Preferably, the tracers are embedded in the proppant material carried by the fracking fluid to more accurately reflect the extent of the resulting fractures. An observation wellbore is then drilled to transect these fractured regions in series. As drilling progresses into each fractured region, the distinctive tracer for that fractured region can be recovered in the observation wellbore. This information can be used to determine the location and extent of the fractured regions. Gaps or regions of overlap between adjacent fractured regions can also be identified. In addition, the observation wellbore allows other types of evaluation of the fractured regions, such as visual inspection of fractures by camera.

RELATED APPLICATION

The present application is based on and claims priority to the Applicant's U.S. Provisional Patent Application 62/964,262, entitled “Method For Evaluating Hydraulic Fracturing,” filed on Jan. 22, 2020.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to the field of hydraulic fracturing of wells. More specifically, the present invention discloses a method for evaluating hydraulic fracturing by detecting tracers in an observation well drilled to transect the fractured regions around a plurality of production wells.

Statement of the Problem

Hydraulic fracturing has been used for many years to enhance oil and gas production from wells. This typically involves perforating the well casing at selected locations in the well, followed by high-pressure injection of fracking fluids and proppant material (e.g., sand or ceramic particles) into the surrounding strata to create fractures that propagate outward from well. The advent of horizontal drilling has allowed wells to run along the plane of the hydrocarbon-bearing strata with a plurality of fractured regions extending into hydrocarbon-bearing strata.

Oil and gas companies attempt to maximize the production of the hydrocarbons by optimizing hydraulic fracturing of the strata surrounding wellbores. Ideally, hydraulic fracturing should maximize the production of hydrocarbons by extending the stimulated reservoir volume to substantially fill the region between adjacent wellbores. But, inter-well communication between adjacent fractured regions can reduce the effectiveness of hydraulic fracture, and leave regions between adjacent wellbores unfractured.

Past efforts to determine the extent of the stimulated reservoir effected by hydraulic fracturing have included geomechanical modelling of fracture propagation into the strata surrounding the well and the interactions between natural and hydraulic fractures. These computer models can be validated to varying degrees using a variety of techniques such as well production logs, microseismic data, radioactive tracers, etc. However, while many attempts have been made to model the interaction between adjacent hydraulically-stimulated wellbores, very little quantifiable evidence has been collected to verify those models. For example, these models had limited capabilities to capture the real world fracture complexity in low permeability reservoir rocks like the Niobrara shale formation.

Tracers have also been used in oil and gas drilling operations to detect the extent of fracturing and track the movement of fracking fluids and subterranean fluids. Conventional tracer materials include salts of various types, such as potassium chloride solutions; inert gases, such as krypton and xenon; and various hydrocarbon compounds. Other tracers rely on radioactivity, such as deuterium oxide or tritium (i.e., heavy water) that can be detected by sensors. The tracer material can be carried by the fracking fluid, which is useful in detecting migration of subterranean fluids. Alternatively, the tracer material can be embedded in the proppant material (e.g., sand or ceramic particles) that is injected into the fractured region during fracturing. Migration of this proppant material is generally limited to the fractured region, which allows the tracer to be used to evaluate the extent of the fractured region.

Solution to the Problem

The present invention proposes a new methodology that will allow quantifiable data to be collected regarding the inter-well communication between multiple wellbores that have been hydraulically stimulated, as well as direct observation of the fracture network. In addition, the present invention allows hydraulic fracture models to be tied to real world data.

The present invention uses an fracture observation well to directly observe the induced hydraulic fracture network through the combination of tagged proppant tracers and image logs. Each observation well can be designed to transect the induced hydraulic fracture network resulting from multiple stages of hydraulic fracturing in a single production well, or it can transect the aligned fracture networks from multiple production wells. The observed hydraulic fracture network can then be validated with distinctive tracers associated with each fractured region as well as geochemical analysis. The observation wells can also be employed to run image logs in the producing wellbores in order to understand regional natural fracturing pre-stimulation.

SUMMARY OF THE INVENTION

This invention provides method for evaluating hydraulic fracturing by placing distinctive tracers in each of the fractured regions around a number of production wellbores. For example, these tracers can by carried in the fracking fluid. Preferably, the tracers are embedded in the proppant material carried by the fracking fluid to more accurately reflect the extent of the resulting fractures. An observation wellbore is then drilled to transect these fractured regions in series. As drilling progresses into each fractured region, the distinctive tracer for that fractured region can be recovered in the observation wellbore. This allows the extent of each fractured region to be determined, including identification of gaps or regions of overlap between adjacent fractured regions. In addition, the observation wellbore allows other types of evaluation of the fractured regions, such as visual inspection of fractures by camera. Preferably, the fractured regions are aligned in a predetermined pattern (e.g., a linear or grid arrangement) to simplify drilling of the observation wellbore.

These and other advantages, features, and objects of the present invention will be more readily understood in view of the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified top-down diagram showing the arrangement of a plurality of production wellbores 10, 20 after fracturing.

FIG. 2 is a simplified top-down diagram showing an observation wellbore 50 transecting the fractured regions 15, 25 adjacent to the production wellbores 10, 20.

FIG. 3 is a simplified diagram corresponding to FIG. 1 showing a vertical cross-section of the production wellbores 10, 20.

FIG. 4 is a simplified diagram corresponding to FIG. 2 showing a vertical cross-section of the observation wellbore 50 transecting the fractured regions 15, 25 adjacent to the production wellbores 10, 20.

DETAILED DESCRIPTION OF THE INVENTION

Turning to FIG. 1, a simplified top-down diagram is provided showing a plurality of production wellbores 10, 20 after fracturing. The resulting fractured regions 15, 25 extend radially outward at intervals along the production wellbores 10, 20. These production wellbores 10, 20 can be drilled, cased, perforated and fractured using conventional technologies that are widely known in the oil and gas industry.

Each wellbore 10, 20 typically has a substantially vertical component extending downward from the surface of the earth to one or more substantially horizontal components running through the hydrocarbon-bearing strata. Secondary horizontal wellbores can drilled to enable more complete fracturing of the hydrocarbon-bearing strata. Ideally, the fractured regions 15, 25 extending outward from the production wellbores 10, 20 should encompass as much of the hydrocarbon-bearing strata as possible to maximize production, without overlap to limit inter-well leakage. The vertical wellbores 10, 20 are often drilled from the surface in a geometric grid pattern (e.g., a triangular or rectangular grid). The horizontal wellbores extending from the lower ends of the vertical wellbores are often drilled parallel to one another, as shown in FIGS. 3 and 4.

After the production wellbores 10, 20 have been drilled and cased, the casing is perforated in a number of stages spaced at intervals along the wellbore. Hydraulic fracturing is used to drive a slurry of fracking fluid and proppant under high pressure into the resulting fractured region 15, 25 adjacent to the wellbore 10, 20. During this fracturing process, a tracer material is introduced into selected fractured regions. For example, the tracer can be carried in the fracking fluid or the proppant. A liquid tracer may have the advantage of penetrating further into the fractured region. However, a liquid tracer remains fluid and may be subject to migration over time due to backflow, hydraulic fracturing of adjacent regions, or leakage with adjacent wellbores, etc. In contrast, attaching the tracer to the proppant limits future migration.

The present invention employs a distinctive tracer for each fractured region of interest to allow subsequent analysis of the material recovered during drilling of the observation well to determine the source of each tracer. For example, a distinctive tracer could be assigned to each individual fractured region 15, 25. However, it may be sufficient to assign different tracers to adjacent fractured regions or adjacent production wellbores, depending on the nature of the study.

FIGS. 2 and 4 show an observation wellbore 50 being drilled from the surface to transect a plurality of fractured regions 15, 25. As this drilling process continues, the drill cuttings and fluids recovered at the wellhead can be monitored and analyzed for the presence of the tracers distinctive of each fractured region 15, 25. As drilling approaches and enters each of the fractured regions 15, 25, the specific tracer assigned to each fractured regions 15, 25 is recovered from the observation wellbore 50. The absence of any tracer indicates the observation wellbore 50 is passing through an unfractured region 40. The presence of multiple tracers may indicate the observation wellbore 50 is passing through a region of overlapping fractured regions 45.

The accompanying drawings illustrate an observation wellbore 50 that that passes through the fractured regions 15, 25 adjacent to multiple productions wellbores 10, 20. Alternatively, the present invention can be employed to evaluate a plurality of fractured regions along a single production wellbore. For example, a plurality of fractured regions can be created in stages at spaced intervals along a single vertical or horizontal wellbore. Preferably, the fractured regions are at least roughly aligned along an axis so that the observation wellbore can be drilled to transect the desired portions of the fractured regions without undue difficulty. This alternative embodiment of the present invention facilitates investigation of the fracture connectivity between adjacent fractured regions.

The observed hydraulic fracture network 15, 25 can be validated with radioactive fluid tracers as well as geochemical analysis. Tracer material can either be carried in the fracking fluids or by proppant material, such as sand or ceramic particles. Liquid tracer are especially useful in tracking the flow of fracking fluids and other subterranean fluids. In contrast, proppant tracers can be employed to mark the extent of the fracture network.

Optionally, the observation wellbore 50 can be drilled to vary its distance from the production wellbores 10, 20 to observe the variation in fracture intensity as a function of distance from the production wellbores 10, 20. The present invention can also be designed to maximize the time that the observation wellbore 50 spends in the induced fracture network 15, 25. For example, as illustrated in FIGS. 2 and 4, the observation wellbore 50 can be designed to transect the fractured region at an angle to ‘walk away’ from the production wellbores 10, 20. This configuration allows observation of the relationship between fracture intensity and distance from the treated lateral, as well as establishing the distance that tagged proppant is placed relative to the treated wellbore.

The observation wells 50 can also be employed to run image logs in the producing wellbores in order to understand regional natural fracturing pre-stimulation. Cuttings can also be collected along the observation wellbores 50, which can be drilled with water-based mud, so that a fluid inclusion analysis can be performed. In particular, this allows the oil contained in each of the different drilling targets to be geochemically fingerprinted and later compared against the produced oil for quantitative production allocation by bench. The produced oil can also be tested in this manner over time to see if the allocated production is changing temporally, which would indicate that bentonites that are originally fractured will heal over time, reducing the inter-well communication. Permanent downhole pressure gauges can also be installed in in each of the producing wellbores. The producing wellbores can be ‘pulsed’ occasionally to see if the communication between wellbores is decreasing over time.

The observation wells 50 can be dedicated wellbores that are drilled for the sole purpose of hydraulic fracture observation and will not produce any hydrocarbons. Alternatively, the observation wells 50 can be later converted to production wells or used for other purposes, such as additional hydraulic fracturing or flooding the fractured regions to increase hydrocarbon production at the production wells.

The present invention can be used to gain a true measure of natural and hydraulic fracture distribution away from the wellbore, and also to gain a true measure of proppant distribution away from the wellbore. In addition, it can be used to evaluate fracture connectivity (overlap 45) between adjacent fractured regions 15, 25 and to evaluate the extent of gaps 40 (unfractured regions) between adjacent fractured regions (e.g., for possible future fracturing or drilling). In turn, this information can be employed to make better-informed decisions regarding well spacing and completion design optimization. The present invention can also be used to validate the results of geomechanical modeling of the fracture network.

The above disclosure sets forth a number of embodiments of the present invention described in detail with respect to the accompanying drawings. Those skilled in this art will appreciate that various changes, modifications, other structural arrangements, and other embodiments could be practiced under the teachings of the present invention without departing from the scope of this invention as set forth in the following claims. 

We claim:
 1. A method for evaluating hydraulic fracturing in fractured regions around a plurality of production wellbores, said method comprising: placing distinctive tracers in each of the fractured regions; drilling an observation wellbore to transect the fractured regions in series; recovering the tracer for each fractured region in the observation wellbore as the observation wellbore is drilled into each fractured region; and determining the extent of the fractured regions based on the recovered tracers in the observation wellbore.
 2. The method of claim 1 further comprising identifying gaps between adjacent fractured regions based on the recovered tracers in the observation wellbore.
 3. The method of claim 1 further comprising identifying regions of overlap between adjacent fractured regions based on the recovered tracers in the observation wellbore.
 4. The method of claim 1 wherein the tracers are embedded in proppant material carried by fracking fluid used in hydraulic fracturing to produce the fractured regions.
 5. The method of claim 1 wherein the fractured regions are aligned in a predetermined pattern.
 6. The method of claim 5 wherein the fractured regions are in a linear arrangement.
 7. The method of claim 1 wherein the observation wellbore is drilled to transect the fractured regions at varying distances from the production wellbores to observe the variation in fracture intensity.
 8. The method of claim 1 further comprising providing a camera in the observation wellbore to enable visual inspection of fractures encountered in the fractured regions.
 9. The method of claim 1 further comprising tracking the flow of subterranean fluids carrying the tracers, based on the recovered tracers in the observation wellbore.
 10. A method for evaluating hydraulic fracturing in fractured regions around a plurality of production wellbores, said method comprising: placing distinctive tracers in each of the fractured regions; drilling an observation wellbore to transect the fractured regions in series; recovering the tracer for each fractured region in the observation wellbore as the observation wellbore is drilled into each fractured region; and identifying gaps and regions of overlap between adjacent fractured regions based on the recovered tracers in the observation wellbore.
 11. The method of claim 10 wherein the tracers are embedded in proppant material carried by fracking fluid used in hydraulic fracturing to produce the fractured regions.
 12. The method of claim 10 wherein the fractured regions are aligned in a predetermined pattern.
 13. The method of claim 10 wherein the observation wellbore is drilled to transect the fractured regions at varying distances from the production wellbores to observe the variation in fracture intensity.
 14. The method of claim 10 further comprising providing a camera in the observation wellbore to enable visual inspection of fractures encountered in the fractured regions.
 15. The method of claim 10 further comprising tracking the flow of subterranean fluids carrying the tracers, based on the recovered tracers in the observation wellbore. 